Since the carbon material has excellent electrical and chemical properties, the application of the carbon material has been extended to the energy storage devices such as lithium ion batteries, and the electrodes of the electric double layer capacitors. However, the theoretical capacity value of the graphite widely used in the anode materials of the lithium ion secondary batteries or the capacitors is low (372 mAh/g), and the further improvement of the capacity of the anode materials using only graphite is a major problem.
In recent years, as materials in place of graphite, materials such as silicon, tin, and aluminum have been noted since the battery capacity is dramatically improved by alloying the materials to lithium electrochemically (for example, refer to Non-Patent Document 1, Patent Document 1). However, in the non-aqueous electrolyte secondary batteries using the lithium alloy and the like as the anode active materials, the alloy materials of the anode active materials are pulverized when repeating charge and discharge cycles, and the characteristics of the anode active material are remarkably lowered. Therefore, the current situation does not yet reach the full-scale practical realization.
Nanonization of metal particles in the composite materials is said to be a very effective mean for suppression of pulverization of the alloy-based active materials associated with charge and discharge. Therefore, since a carbon composite carrying metal particles of a nanometer order is expected as a next-generation functional material with superior characteristics, research and development with regard to the method for supporting nano-metal in the composite materials have also attracted attention.
The methods for synthesizing the composite materials by mixing fine particles of the metal phase and the carbon phase (fibrous, tubular, porous, etc.) are widely reported (for example, refer to Patent Documents 2 to 5). The Patent Documents 2 to 5 relate to the methods for producing composite particles by using a net-like structure comprising resin carbon materials or nanofibers, etc., and by surrounding-coating fine particles such as a metal, a semi-metal, and an alloy. Since the composite materials obtained by these methods are in a state in which the metal phases and the carbon phases are simply mixed, or they simply adhere, there is a defect in which the metal particles are easily desorbed from the carbon phase.
As a method for effectively compounding metal tin and carbon material, for example, a method for synthesizing tin-carbon composite by calcinating a gel comprising resorcinol-benzene-1,3-formaldehyde and metal tin organic compounds in an argon atmosphere, has been proposed (for example, refer to Non-Patent Document 2). The metal tin in the composite obtained by this method is a nanoparticle having an average particle size of about 36 nm, and they are all included in the carbon material. However, the form of the obtained carbon composite material is entirely irregular, a large variation in the distribution of tin particles is also observed, and it is one that lacks homogeneity.
In addition, a method for producing tin-carbon composite particles by heat treatment after metal ions are absorbed in ion-exchange resins, has been proposed (for example, refer to Patent Document 6). In Patent Document 6, the cation exchange resins which can be ion-exchanged with the metal ion are used, among these, it is described that the cation exchange resins to be used may be one to which the ion-exchange group such as a phenolic hydroxy group, a carboxy group, or a sulfonic acid group binds. Although preparation of the carbon composite material containing fine particles of the metal compound is possible by this method, it is difficult to control the morphology of the composite particles by this method, or to control the particle size of the fine particles of the metal compound at several nano-level. When observing the photographs shown in the Patent Document 6, ones whose particle size is more than 50 nm are included, and it is apparent that its distribution is not controlled.
Non-Patent Document 3 is a review of “the tin-based anode material”, which was published in 2011. It refers to the current state of the production method of the tin-carbon composite materials, and charge & discharge characteristics of the anode materials using each composite material are also summarized in detail. The carbon layer coating onto the particles, metal supporting on the carbon fiber or in a nanotube, and the like, are described in a number of research examples which are effective methods for stabilizing the metal tin. However, research report of the sheet-like metal tin-carbon composite that contains metal tin particles has not been described.
The constitution of the sheet-like composite that fully encloses metal tin nanoparticles by carbon materials, well exhibits the protective effect of carbon to the metal tin phase. In addition, it is estimated that it is possible to completely find the size effect of the metal tin nanoparticles by thoroughly eliminating the coarse particles of metal tin in the carbon composite materials. Furthermore, the sheet-like form is advantageous for film-forming properties of the powders, and it is considered to be led to the improvement of the electronic transitions and conductivity. Therefore, a material having such a structure is expected to be applied in a wide range as a new functional material.